Abstract: A high strength cover for an underground utility box of the type that is normally installed on streets and driveways and is subjected to substantial vehicle loads. The cover structure includes a fixed rectangular frame the upper edge of which is installed substantially flush with the ground level. Below the ground level the frame defines bearing surfaces throughout the periphery of the frame, the inner edge of the bearing members defining the access opening. Two structural frameworks, dimensioned to be, in combination, substantially coextensive with the access opening are provided to span the bearing surfaces. The frameworks have sufficient vertical dimensions to afford the necessary strength. The frameworks are hinged for pivotal movement about axes along two opposite ends of the frame so that they can be moved to an upright position to afford access to the box.

Abstract: A system for producing a reverse blast of air to dislodge from an air-filter element contaminents that are captured on the surface of the element in response to forward flow through the element. A manifold pipe spanning one or more openings at the outlet end of the filter elements and a nozzle supported on the manifold pipe in alignment with each element opening. Associated with the nozzle and secured to the manifold pipe for rotation in unison therewith is a venturi which, when rotated into an active position substantially covers the opening so that when air is supplied through the manifold pipe to the nozzle, the reverse blast is effective in dislodging contaminents on the exterior of the element. The manifold pipe is rotatable to an inactive position at which the venturi and nozzle are virtually clear of the outlet openings so as to avoid impedence to air flow through the filter.